You have the key for Prime Arena testing. What's next?
Open your library in Steam ->
Click on the button in the bottom left that says "add a game..." ->
Choose "Activate a Product on Steam..." -> Click "Next" ->
Then "I Agree" -> Enter the code that you received.
After that, the game will appear in your library and you will be able to install it. In the settings you can also choose between Russian and English.